1. Field of the Invention
The present invention relates to an air conditioner with a refrigerating cycle employing a non-azeotropic mixture of high and low boiling coolants, and particularly, to optimizing the operation of the refrigerating cycle.
2. Description of the Prior Art
Heat pump air conditioners employ the atmosphere as a heat source for achieving heating and cooling operations. A refrigerating cycle employed by the heat pump air conditioners essentially involves a compressor for discharging a high-temperature high-pressure gaseous coolant, a four-way valve for changing the direction of the coolant according to heating and cooling operations, an outdoor heat exchanger serving as an evaporator during the heating operation and as a condenser during the cooling operation, a throttle mechanism such as an expansion valve to reduce the pressure of the coolant, and an indoor heat exchanger serving as a condenser during the heating operation and as an evaporator during the cooling operation.
These components are arranged in outdoor and indoor units. The outdoor unit may accommodate the compressor, an inverter to change the operation frequency of the compressor, the throttle mechanism, the four-way valve, the outdoor heat exchanger, and an outdoor fan. The indoor unit may accommodate the indoor heat exchanger and an indoor fan.
During the heating operation, the coolant flows through the outdoor heat exchanger to pump up heat from the atmosphere and through the indoor heat exchanger to discharge the heat into an atmosphere in a room. During the cooling operation, the coolant flows through the indoor heat exchanger to pump up heat from the room atmosphere and through the outdoor heat exchanger to abandon the heat into the atmosphere.
The performance of the air conditioner is dependent on the capacities of the outdoor and indoor heat exchangers. To meet present requirements of compactness and high performance, efficient small-sized heat exchangers and low-noise fans are being developed. Some heat exchangers employ small diameter coolant pipes to reduce the size of their indoor units and improve the efficiency thereof. The outdoor units are also required to be compact. It is not so successful as for the indoor units to reduce the size of the outdoor units due to the problem of frosting during a heating operation. Techniques applicable to the indoor heat exchangers such as forming slits on heat radiation fins of indoor heat exchangers and arranging the indoor heat exchangers In three or more stages are not applicable to the outdoor heat exchangers due to the problem of frosting.
In view of protection of global environments such as the ozone layer and prevention of global warming, the air conditioners are required to employ replacements of fluorocarbon-based coolant R22. The replacements are mostly non-azeotropic mixtures having similar cycle temperature and pressure as R22. The non-azeotropic mixtures are usually composed of high and low boiling coolants and show a large temperature gradient between gas and liquid phases. Accordingly, the non-azeotropic mixtures have poor heat transfer efficiency.
During the heating operation to pump up heat from the atmosphere, the non-azeotropic mixtures may freeze at an inlet of the outdoor heat exchanger. An effective temperature difference between an ambient temperature and the evaporation temperature of the mixtures in the outdoor heat exchanger is small to lower the heating capacity of the outdoor heat exchanger if the ambient temperature is low. During the cooling operation, the mixtures may freeze around the inlet of the indoor heat exchanger if the ambient temperature is low, to deteriorate the cooling capacity of the indoor heat exchanger.
To improve the air-conditioning capacity, some refrigerating cycles change the ratio between the high and low boiling coolants of the non-azeotropic mixture. This technique accumulates a liquid phase portion of the high boiling coolant in a tank, to increase the ratio of the high-performance low boiling coolant to the high boiling coolant.
After the ratio between the high and low boiling coolants is changed, the basic control conditions of the air conditioner such as the operation frequency of the compressor are usually unchanged. This results in destabilizing the operation of the refrigerating cycle because the change in the ratio between the high and low boiling coolants changes the cycle temperature and pressure of the coolant mixture.